1. Field of the Invention
The present invention relates generally to a common channel communication apparatus and method for a CDMA communication system, and in particular, to an apparatus and method for communicating data over a common packet channel in an asynchronous CDMA communication system.
2. Description of the Related Art
An asynchronous CDMA communication system, such as the UMTS (Universal Mobile Telecommunications System) W-CDMA (Wideband Code Division Multiple Access) communication system, which is a future mobile communication system, uses a random access channel (RACH) and a common packet channel (CPCH) for an uplink (or reverse) common channel.
FIG. 1 is a diagram for explaining how to transmit and receive a traffic signal over the RACH, which is one of the conventional asynchronous uplink common channels. In FIG. 1, reference numeral 151 indicates a signal transmission procedure of an uplink channel, which can be the RACH. Further, reference numeral 111 indicates an access preamble-acquisition indicator channel (AICH), which is a downlink (or forward) channel. The AICH is a channel over which a UTRAN (UMTS Terrestrial Radio Access Network) receives a signal transmitted from the RACH and responds to the received signal. The signal transmitted by the RACH is called an “access preamble” (AP), which is created by randomly selecting one of the signatures for the RACH.
The RACH selects an access service class (ASC) according to the type of transmission data, and acquires from the UTRAN the right of using a channel using a RACH sub-channel group and an AP defined in the ASC.
Referring to FIG. 1, a user equipment (UE) transmits an AP 162 of specific length using the RACH and then awaits a response from the UTRAN. If there is no response from the UTRAN for a predetermined time, the UE increases transmission power by a specific level as represented by 164 and retransmits the AP at the increased transmission power. Upon detecting the AP transmitted over the RACH, the UTRAN transmits a signature 122 of the detected AP over the downlink AICH. After transmitting the AP, the UE determines whether the transmitted signature is detected from the AICH signal that the UTRAN has transmitted in response to the AP. In this case, if the signature used for the AP transmitted over the RACH is detected, the UE judges that the UTRAN has detected the AP, and transmits a message over the uplink access channel.
Otherwise, upon failure to detect the transmitted signature from the AICH signal that the UTRAN has transmitted within a set time Tp-AI after transmission of the AP 162, the UE judges that the UTRAN has failed to detect the preamble, and retransmits the AP after a lapse of a preset time. As represented by reference numeral 164, the AP is retransmitted at transmission power increased by ΔP(dB) from the transmission power at which the AP was previously transmitted. The signature used to create the AP is randomly selected from the signatures defined in the ASC selected by the UE. Upon failure to receive the AICH signal using the transmitted signature from the UTRAN after transmission of the AP, the UE changes, after a lapse of a set time, the transmission power and signature of the AP and repeatedly performs the above operation. In the process of transmitting the AP and receiving the AICH signal, if the signature transmitted by the UE itself is received, the UE spreads, after a lapse of a preset time, a RACH message 170 with a scrambling code for the signature, and transmits the spread RACH message using a predetermined channelization code at transmission power for a preamble to which the UTRAN has responded with the AICH signal (i.e., at initial power for an uplink common channel message).
As described above, by transmitting the AP using the RACH, it is possible for the UTRAN to efficiently detect the AP and to readily set the initial power of an uplink common channel message. However, since the RACH is not power controlled, it is difficult to transmit packet data, which has a long transmission time because the UE has a high data rate or has a large amount of transmission data. In addition, since the channel is allocated through one AP_AICH (Access Preamble-Acquisition Indicator Channel), the UEs that have transmitted the AP using the same signature will use the same channel. In this case, the data transmitted by the different UEs collide with one another, so that the UTRAN cannot receive the data.
To solve this problem, a method for suppressing a collision between the UEs while power controlling the uplink common channel has been proposed for the W-CDMA system. This method is applied to a common packet channel (CPCH). The CPCH enables power control of the uplink common channel, and shows a higher reliability as compared with the RACH in allocating the channel to different UEs. Thus, the CPCH enables the UE to transmit a data channel of a high rate for a predetermined time (from several tens to several hundreds of ms). Further, the CPCH enables the UE to rapidly transmit an uplink transmission message, which is smaller in size than a specific value, to the UTRAN without using a dedicated channel.
In order to establish the dedicated channel, many related control messages are exchanged between the UE and the UTRAN, and a long time is required in transmitting and receiving the control messages. Therefore, exchanging many control messages in transmitting data of a comparatively small size of several tens to several hundreds of ms, becomes a needlessly large amount of overhead. Thus, it is more effective to use the CPCH, when transmitting data of a small size.
However, since several UEs transmit preambles using several signatures in order to acquire the right of using the CPCH, there may occur a collision between the CPCH signals from the UEs. To avoid this phenomenon, a method is needed for allocating the right of using the CPCH to the UEs.
The asynchronous mobile communication system uses a downlink scrambling code to distinguish the UTRANs, and uses an uplink scrambling code to distinguish the UEs. Further, the channels transmitted from the UTRAN are distinguished using an orthogonal variable spreading factor (OVSF) code, and the channels transmitted by the UE are also distinguished using the OVSF code.
Therefore, the information required by the UE to use the CPCH, includes a scrambling code used for a message part of the uplink CPCH channel, an OVSF code used for the message part (UL_DPCCH) of the uplink CPCH, an OVSF code used for a data part (UL_DPDCH) of the uplink CPCH, a maximum data rate of the uplink CPCH, and a channelization code for a downlink dedicated channel (DL_DPCCH) used for power control of the CPCH. The above information is typically required when establishing a dedicated channel between the UTRAN and the UE. Further, the above information is transmitted to the UE through transmission (overhead) of signaling signals before establishment of the dedicated channel. However, since the CPCH is a common channel rather than a dedicated channel, the above information is conventionally represented by a combination of the signatures used in the AP and the CPCH sub-channels to which the sub-channel concept used in the RACH is introduced, in order to allocate the information to the UE.
FIG. 2 shows a signal transmission procedure of the downlink and uplink channel signals according to the prior art. In FIG. 2, in addition to the method used for the RACH for transmitting the AP, a collision detection preamble (CD_P) is used to prevent a collision between CPCH signals from the different UEs.
In FIG. 2, reference numeral 211 indicates an operating procedure of an uplink channel performed when the UE operates to be allocated the CPCH, and reference numeral 201 indicates an operating procedure of the UTRAN to allocate the CPCH to the UE. In FIG. 2, the UE transmits an AP 213. For a signature constituting the AP 213, it is possible to use a selected one of the signatures used in the RACH or to use the same signature, and the signature can be distinguished using the different scrambling codes. The signature constituting the AP is selected by the UE based on the above-stated information, and this is what is different from the method where the RACH randomly selects the signature. That is, to each signature are mapped an OVSF code to be used for the UL_DPCCH, an OVSF code to be used for the UL_DPDCH, an OVSF code to be used for the UL_Scrambling code and DL_DPCCH, the maximum frame number, and a data rate. Therefore, in the UE, selecting one signature is equivalent to selecting four kinds of the information mapped to the corresponding signature. In addition, the UE examines a status of the CPCH channel which can be presently used in the UTRAN to which the UE belongs, through a CPCH status indicator channel (CSICH) transmitted using an ending part of the AP_AICH before transmitting the AP. Thereafter, the UE transmits the AP over the CSICH after selecting the signatures for the channel to be used out of the CPCHs which can be presently used. The AP 213 is transmitted to the UTRAN at initial transmission power set by the UE. In FIG. 2, if there is no response from the UTRAN within a time 212, the UE retransmits the AP 215. The retransmission number of the AP and the waiting time 212 are set before a process for acquiring the CPCH channel is started, and the UE stops the CPCH channel acquisition process when the retransmission number exceeds a set value.
Upon receipt of the AP 215, the UTRAN compares the received AP with the APs received from other UEs. Upon selecting the AP 215, the UTRAN transmits AP_AICH 203 as ACK after a lapse of a time 202. There are several criteria based on which the UTRAN compares the received APs to select the AP 215. For example, the criteria may correspond to a case where the CPCH, for which the UE has requested the UTRAN through the AP, is available, or a case where the receiving power of the AP received by the UTRAN satisfies the minimum receiving power requested by the UTRAN. The AP_AICH 203 includes a value of the signature constituting the AP 215 selected by the UTRAN. If the signature transmitted by the UE itself is included in the AP_AICH 203 received after transmitting the AP 215, the UE transmits a collision detection preamble (CD_P) 217 after a lapse of a time 214. A reason for transmitting the CD_P 217 is to prevent a collision between transmission channels from the UEs. That is, many UEs belonging to the UTRAN may request the right of using the same CPCH by simultaneously transmitting the same AP to the UTRAN, and as a result, the UEs receiving the same AP_AICH may try to use the same CPCH, thereby causing a collision. Each of the UEs which have simultaneously transmitted the same AP, selects the signature to be used for the CD_P and transmits the CD_P. Upon receipt of the CD_Ps, the UTRAN can select one of the received CD_Ps and respond to the selected CD_P. For example, a criterion for selecting the CD_P can be a receiving power level of the CD_P received from the UTRAN. For the signature constituting the CD_P 217, one of the signatures for the AP can be used, and it can be selected in the same manner as in the RACH. That is, it is possible to randomly select one of the signatures used for the CD_P and transmit the selected signature. Alternatively, only one signature can be used for the CD_P. When there is only one signature used for the CD_P, the UE selects a randomized time point in a specific time period to transmit the CD_P at the selected time point.
Upon receipt of the CD_P 217, the UTRAN compares the received CD_P with the CD_Ps received from other UEs. Upon selecting the CD_P 217, the UTRAN transmits a collision detection indicator channel (CD_ICH) 205 to the UEs after a lapse of a time 206. Upon receipt of the CD_ICH 205 transmitted from the UTRAN, the UEs check whether a value of the signature used for the CD_P transmitted to the UTRAN is included in the CD_ICH 205, and the UE, for which the signature used for the CD_P is included in the CD_ICH 205, transmits a power control preamble (PC_P) 219 after a lapse of a time 216. The PC_P 219 uses an uplink scrambling code determined while the UE determines a signature to be used for the AP, and the same channelization code (OVSF) as a control part (UL_DPCCH) 221 during transmission of the CPCH. The PC_P 219 is comprised of pilot bits, power control command bits, and feedback information bits. The PC_P 219 has a length of 0 or 8 slots. The slot is a basic transmission unit used when the UMTS system transmits a physical channel, and has a length of 2560 chips when the UMTS system uses a chip rate of 3.84 Mcps (chips per second). When the length of the PC_P 219 is 0 slot, the present radio environment between the UTRAN and the UE is good, so that the CPCH message part can be transmitted at the transmission power at which the CD_P was transmitted, without separate power control. When the length of the PC_P 219 is 8 slots, it is necessary to control transmission power of the CPCH message part.
The AP 215 and the CD_P 217 may use the scrambling codes which have the same initial value but have different start points. For example, the AP can use 0th to 4095th scrambling codes of length 4096, and the CD_P can use 4096th to 8191st scrambling codes of length 4096. The AP and CD_P can use the same part of the scrambling code having the same initial value, and such a method is available when the W-CDMA system separates the signatures used for the uplink common channel into the signatures for the RACH and the signatures for the CPCH. For the scrambling code used for the PC_P 219, are used the 0th to 21429th values of the scrambling code having the same initial value as the scrambling code used for AP 215 and CD_P 217. Alternatively, for the scrambling code for the PC_P 219, a different scrambling code can also be used which is mapped one-to-one with the scrambling code used for AP 215 and CD_P 217.
Reference numerals 207 and 209 denote a pilot field and a power control command field of a dedicated physical control channel (DL_DPCCH) out of a downlink dedicated physical channels (DL_DPCHs), respectively. The DL_DPCCH can use either a primary downlink scrambling code for distinguishing the UTRANs or a secondary scrambling code for expanding the capacity of the UTRAN. For a channelization code OVSF to be used for the DL_DPCCH, is used a channelization code which is determined when the UE selects the signature for the AP. The DL_DPCCH is used when the UTRAN performs power control on the PC_P or CPCH message transmitted from the UE. The UTRAN measures receiving power of a pilot field of the PC_P 219 upon receipt of the PC_P 219, and controls transmission power of the uplink transmission channel transmitted by the UE, using the power control command 209. The UE measures power of a DL_DPCCH signal received from the UTRAN to apply a power control command to the power control field of the PC_P 219, and transmits the PC_P to the UTRAN to control transmission power of a downlink channel incoming from the UTRAN.
Reference numerals 221 and 223 denote a control part UL_DPCCH and a data part UL_DPDCH of the CPCH message, respectively. For a scrambling code for spreading the CPCH message of FIG. 2, a scrambling code is used which is identical to the scrambling code used for the PC_P 219. For the used scrambling code, are used 0th to 38399th scrambling codes of length 38400 in a unit of 10 ms. The scrambling code used for the message of FIG. 2 can be either a scrambling code used for the AP 215 and the CD_P 217, or another scrambling code which is mapped on a one-to-one basis. The channelization code OVSF used for the data part 223 of the CPCH message is determined according to a method previously appointed between the UTRAN and the UE. That is, since the signature to be used for the AP and the OVSF code to be used for the UL_DPDCH are mapped, the OVSF code to be used for the UL_DPDCH is determined by determining the AP signature to be used. For the channelization code used by the control part (UL_DPCCH) 221, a channelization code is used which is identical to the OVSF code used by the PC_P. When the OVSF code to be used for the UL_DPDCH is determined, the channelization code used by the control part (UL_DPCCH) 221 is determined according to an OVSF code tree structure.
Referring to FIG. 2, the prior art enables power control of the channels in order to increase efficiency of the CPCH, which is the uplink common channel, and decreases the chance of a collision between uplink signals from the different UEs, by using the CD_P and the CD_ICH. However, in the prior art, the UE selects all the information for using the CPCH and transmits the selected information to the UTRAN. This selecting method can be performed by combining a signature of the AP, a signature of the CD_P and the CPCH sub-channel, transmitted from the UE. In the prior art, even though the UE requests allocation of the CPCH channel required by the UTRAN by analyzing a status of the CPCH, which is presently used in the UTRAN, by using the CSICH, that the UE previously determines all the information required for transmitting the CPCH and transmits the determined information will cause a limitation on allocation of the CPCH channel and a delay in acquiring the channel.
The limitations on allocation of the CPCH channel are as follows. Although there exist several available CPCHs in the UTRAN, if the UEs in the UTRAN require the same CPCH, the same AP will be selected. Although the same AP_AICH is received and the CD_P is transmitted again, the UEs which transmitted the non-selected CD_P should start the process for allocating the CPCH from the beginning. In addition, although the CD_P selecting process is performed, many UEs still receive the same CD_ICH, increasing a probability that a collision will occur during uplink transmission of the CPCH. Further, although the CSICH is checked and the UE requests the right of using the CPCH, all the UEs in the UTRAN which desire to use the CPCH receive the CSICH. Therefore, even though an available channel is required out of the CPCHs, there is a case where several UEs simultaneously requests channel allocation. In this case, the UTRAN cannot but allocate the CPCH requested by the UE, even though there are other CPCHs which can be allocated.
With regard to a delay in acquiring the channel, when the case occurs which has been described with reference to the limitations on allocation of the CPCH channel, the UE should repeatedly perform the CPCH allocation request to allocate the desired CPCH channel. When there is used a method for transmitting the CD_P at a given time for a predetermined time using only one signature for the CD_P introduced to reduce the complexity of the system, it is not possible to process the CD_ICH of other UEs while transmitting and processing the CD_ICH of one UE.
In addition, the prior art uses one uplink scrambling code in association with one signature used for the AP. Thus, whenever the CPCH used in the UTRAN increases in number, the uplink scrambling code also increases in number, causing a waste of the resources.